The present invention relates generally to photolithography as applied to microcircuit fabrication and, more specifically, to a method for correcting misalignment between a reticle and a stage in a MICRASCAN step-and-repeat exposure system.
The fabrication of integrated circuits requires a number of photolithographic fabrication steps to form the intricate circuitry that is required for a particular chip design. The process comprises sequentially creating multiple layers on the chip, each with intricate circuit patterns. A number of methods are known for creating the various patterned layers, including a widely used process known as photolithography. According to this process, a photoresist layer is first applied over the surface on which it is desired to form a pattern. An image of a desired circuit pattern is next projected onto the photoresist layer. Typically, the photoresist layer hardens where exposed and becomes resistant to certain resist-removing techniques. The unexposed resist is next removed, and the underlying material is then uncovered and subsequently etched away. The expose resist is then removed and the underlying pattern uncovered for use.
Typically, more than one layer of circuitry is required in the manufacture of integrated circuits. Thus, the above process may be repeated more than once, creating multiple circuit layers separated by insulating layers and interconnected through vias, as needed. All of these layers must be accurately aligned, one on top of another.
The pattern projected onto the photoresist for each layer is carried onto a reticle, each reticle carrying the pattern for one layer. The reticle usually comprises a finely etched pattern of metal (such as chrome) on a glass or quartz substrate.
In a commercial manufacturing process, more than one chip is produced simultaneously on a single object, such as a silicon wafer. Typically a pattern is exposed onto each chip through a step-and-repeat process in which the wafer or the reticle is moved between exposures to bring a new chip in place for exposure. This process requires precise alignment between the reticle and the target area of the object.
An early approach to solving the alignment problem was contact alignment in which the reticle was placed in actual contact with the object and the light source was a collimated ultra violet (UV) source illuminating the entire image on the reticle. With the need for finer and finer patterns, however, it soon became essential to remove the reticle from contact with the object surface so that the image of the reticle could be reduced by projection onto the photoresist layer.
U.S. Pat. No. 4,068,947 issued to Buckley, et al. describes a system for a projection-type exposure and aligning system originally developed and marketed by the Perkin-Elmer Corporation under the trademark MICRALIGN. In the MICRALIGN system, a wafer, on which a plurality of chips will be manufactured, and the reticle, with the appropriate pattern, are mounted on a carriage. A light source illuminates a selected area of the reticle and focuses a portion of the image onto the photoresist. The carriage is moved so that the illuminated region is scanned across the reticle and the wafer. Unfortunately, the MICRALIGN system used a 1:1 projection ratio and did not provide for image reduction.
A different approach, known as step-and-repeat, uses a stepper to sequentially project a full image of the reticle onto each chip on the wafer. Although this method is simple, provides good alignment, and uses reducing optics, the demands for a full-field optical system become more stringent as feature sizes shrink and chip sizes increase.
The next step was the development of equipment such as the MICRASCAN system marketed by SVG Lithography Systems Inc. (hereinafter xe2x80x9cSVGxe2x80x9d). SVG is a subsidiary of Silicon Valley Group Inc. which, in turn, is a successor to the lithography business of Perkin-Elmer. In the MICRASCAN system, each chip is exposed by scanning onto the wafer a demagnified image of a slit moving over the reticle, from chip-to-chip on the wafer. This system is similar in concept to the MICRALIGN system, but substantially more complex as it requires synchronous scanning of the wafer and reticle relative to the optical system, with the wafer moving slower than the reticle to obtain image reduction. This advantage is achieved in the MICRASCAN system using two interferometrically controlled air bearing stages driven by linear motors.
Although the MICRASCAN system has proven extremely useful in the commercial fabrication of integrated circuits, it suffers from a need for frequent alignment calibration between the reticle and the stage on which the wafer is mounted to correct for drift due to temperature change, a procedure referred to as xe2x80x9creticle refresh.xe2x80x9d Deciding when to perform reticle refresh is important as the procedure interrupts the normal production progress, resulting in lost time. Drift appears to be large during the early operation of the equipment and as time passes it becomes negligent. It is thus important to know when to implement reticle refresh.
SVG has partially addressed the drift-refresh problem by monitoring the reticle stage front plate temperature. A good correlation has been found between the drift in shift X of the reticle and the temperature change of one of two temperature sensors provided by SVG on the MICRASCAN system. Software is used to read the temperature at each wafer alignment and apply it to either a default calibration coefficient or to an externally set coefficient. The resulting values are applied to the reticle refresh shift X and shift Y solution for a current wafer, and the sum total compensation since the last reticle refresh is compared to a maximum correction limit. If this limit is exceeded, a command is issued to perform a reticle refresh cycle.
Experience has shown that, during typical production of integrated circuits, reticle refresh will occur every three to four wafers or, in terms of time, every four to five minutes. Reticle refresh takes about a minute to perform, which results in loss of production time and may also result in progressively larger alignment errors in the wafers between refresh cycles.
There is a need, therefore, for a system that can be implemented on the MICRASCAN system that will reduce the need for reticle refresh. There is also a need for a system that can be implemented on the MICRASCAN system that further reduces alignment errors and improves productivity.
To meet these and other needs, and in view of its purposes, the present invention provides a method for correcting misalignment between a reticle and a stage in a MICRASCAN step-and-repeat exposure system. In such a system, the reticle and the stage lie in parallel X-Y planes and there are at least two temperature sensors associated with the reticle, sensor 1 and sensor 2, sensor 1 having a sensor 1 temperature output and sensor 2 having a sensor 2 temperature output. The method comprises the steps of:
(a) correlating reticle X shift as a function of the sensor 1 temperature output;
(b) correlating reticle Y shift as a function of the sum of the sensor 1 and sensor 2 temperature outputs;
(c) calculating a coefficient 1 equal to the slope of the correlation obtained in step (a);
(d) calculating a coefficient 2 equal to the slope of the correlation obtained in step (b);
(e) loading a first object on the stage;
(f) aligning the reticle relative to the stage and determining a first X shift (X1 shift) of the reticle relative to the stage and a first Y shift (Y1 shift) of the reticle to the stage;
(g) aligning the first object relative to the stage;
(h) obtaining a first sensor 1 temperature output (T1S1) and a first sensor 2 temperature output (T1S2);
(i) exposing the first object to radiation through the reticle;
(j) unloading the first object;
(k) loading a second object on the stage;
(l) obtaining a second sensor 1 temperature output (T2S1) and a second sensor 2 temperature output (T2S2);
(m) estimating a second X shift (X2 shift) and a second Y shift (Y2 shift) of the reticle relative to the stage using the relationships:
X2 shift=(T2S1xe2x88x92T1S1)*coefficient 1;
Y2 shift=[(T2S1xe2x88x92T1S1)+(T2S2xe2x88x92T1S2)]*coefficient 2;
(n) estimating a total X shift (Xtotal shift) and a total Y shift (Ytotal shift) using the relationships:
Xtotal shift=X1 shift+X2 shift;
Ytotal shift=Y1 shift+Y2 shift;
and
(o) correcting misalignment between the reticle and the stage using the Xtotal shift and the Ytotal shift.
The invention also provides a method for correcting reticle X shift and reticle Y shift between a reticle and a stage in a MICRASCAN step-and-repeat exposure system. In such a system, there are at least two temperature sensors associated with the reticle, sensor 1 and sensor 2, sensor 1 having a sensor 1 temperature output and sensor 2 having a sensor 2 temperature output. The method comprises the steps of:
(a) estimating an X shift (Xtotal shift) by multiplying a coefficient 1 representing a reticle X shift per degree of temperature output of sensor 1 with a difference of the temperature output of sensor 1 between a first and a second time;
(b) estimating a Y shift (Ytotal shift) by multiplying a coefficient 2 representing a reticle Y shift per degree of the sum of temperature outputs of sensors 1 and 2 with the difference between the sum of the temperature outputs of sensor 1 and sensor 2 between the first and the second time; and
(c) correcting reticle X shift and reticle Y shift between the reticle and the stage using the calculated X shift (Xtotal shift) and Y shift (Ytotal shift).
Typically, the object is a wafer of the type used in the production of integrated circuits, such as a silicon wafer. The object usually comprises a plurality of adjacent distinct exposure fields that are exposed to radiation sequentially through the reticle.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention.